CentOS / Redhat : Configure CentOS as a Software Router with two interfaces


Linux can be easily configured to share an internet connection using iptables. All you need to have is, two network interface cards as follows:

a) Your internal (LAN) network connected via eth0 with static ip address 192.168.0.1

b) Your external WAN) network is connected via eth1 with static ip address 10.10.10.1  ( public IP provided by ISP )
Please note that interface eth1 may have public IP address or IP assigned by ISP. eth1 may be connected to a dedicated DSL / ADSL / WAN / Cable router:

Step # 1: Enable Packet Forwarding

Login as the root user. Open /etc/sysctl.conf file
# vi /etc/sysctl.conf

Add the following line to enable packet forwarding for IPv4:
net.ipv4.conf.default.forwarding=1

Save and close the file. Restart networking:
# service network restart

Step # 2: Enable IP masquerading


In Linux networking, Network Address Translation (NAT) or Network Masquerading (IP Masquerading) is a technique of transceiving network traffic through a router that involves re-writing the source and/or destination IP addresses and usually also the TCP/UDP port numbers of IP packets as they pass through. In short, IP masquerading is used to share the internet connection.

Share internet connection

To share network connection via eth1, enter the following rule at command prompt (following useful for ppp0 or dial up connection):


# service iptables stop

# iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE

# service iptables save

# service iptables restart

Make sure Iptables runs on boot

# chkconfig iptables on


Open your Windows / Mac / Linux computer networking GUI tool and point router IP to 192.168.0.1 (eth0 Linux IP). You also need to setup DNS IP such as 8.8.8.8 or to your local DNS server IP. You should now able to ping or browse the internet:

# ping google.com


Client Side Configuration

Now all you have to do is set the IP 192.168.0.1 as gateway on all your client machines in your network. You can enter  your default gateway in the file /etc/sysconfig/network as:- 
GATEWAY=192.168.0.1
or you can also put the same entry in your interface specific file at /etc/sysconfig/network-scripts/ifcfg-eth0 in case of RHEL/Centos or at /etc/network/interfaces in case of ubuntu.



4 comments:

  1. Excellent, worked great! I used your instructions to implement something a bit different, however.

    We already have a hardware router in place, and I wanted to set up a transparent proxy for all the users without having to deploy it through GPO. So I used your steps to configure the CentOS VM as the default gateway with the transparent squid proxy, then changed the default gateway on my DHCP server.

    Now everyone goes through the CentOS VM (gateway with squid), and this one goes out through our hardware router.

    Since we already have a firewall in place, I may disable iptables or set it on "allow-all" mode to avoid having to configure everything twice, but I'm not sure about this yet. Still testing...

    Thanks!

    ReplyDelete
    Replies
    1. Glad it helped.

      As a security/web policy, you can also block all web request on your router except those coming from squid machine.

      or you can have your router in different network & configure only squid machine to use that.

      Regds
      Balvinder

      Delete
  2. Hi there can this setup be done on AWS amazon using 3 redhats?

    ReplyDelete
  3. sry i don't get it- which is the interface the other computers are going to see and which is the one connected to the modem/internet?

    ReplyDelete